Obesity has reached epidemic proportions worldwide and is associated with chronic diseases such as type 2 diabetes mellitus, cardiovascular diseases, hepatosteatosis, and cancer. Obesity develops as a result of energy intake that exceeds energy expenditure, leading to a net storage of excess calories in the form of fat in adipose tissue. Obesity is metabolically linked with type 2 diabetes (insulin resistance) and hepatosteatosis, the latter of which can lead to steatohepatitis, hepatocarcinogenesis and liver failure. Thus, a pharmaceutical approach that suppresses appetite, blocks dietary fat absorption, induces fat mobilization, or increases metabolism would be ideal in the treatment of obesity and related metabolic disorders.
Farnesoid X Receptor (FXR) is an orphan nuclear receptor initially identified from a rat liver cDNA library (Forman, et al., Cell 81:687-693, 1995) that is most closely related to the insect ecdysone receptor. FXR is a member of the nuclear receptor superfamily of transcription factors that includes receptors for the steroid, retinoid, and thyroid hormones (Mangelsdorf, et al., Cell 83:841-850, 1995). Northern blotting and in situ hybridization analysis showed that FXR is most abundantly expressed in the liver, intestine, kidney, and adrenal (B. M. Forman, et al., Cell 81:687-693.1995; Seol, et al., Mol. Endocrinol. 9:72-85, 1995). FXR is a ligand-activated nuclear receptor that binds to DNA as a heterodimer with the retinoic acid receptor α (RXRα) that is activated by the vitamin A derivative 9-cis retinoic acid. The FXR/RXRα heterodimer preferentially binds to response elements composed of two nuclear receptor half sites of the consensus AG(G/T)TCA organized as an inverted repeat and separated by a single nucleotide (IR-1 motif) (Forman, et al., Cell 81:687-693, 1995). An early report showed that rat FXR is activated by micromolar concentrations of farnesoids such as farnesol and juvenile hormone thus accounting for the original name (Forman, et al., Cell 81:687-693, 1995). However, these compounds were weak ligands and also failed to activate the corresponding mouse and human FXR, leaving the nature of the endogenous FXR ligand in doubt. However, several naturally-occurring bile acids were found to bind to and activate FXR at physiological concentrations (Makishima, et al., Science 284:1362-1365, 1999; Parks, et al., Science 284:1365-1368, 1999; Wang et al., Mol. Cell 3:543-553, 1999; PCT WO 00/37077, published Jun. 29, 2000). The bile acids that serve as FXR ligands include chenodeoxycholic acid (CDCA), deoxycholic acid (DCA), lithocholic acid (LCA), and the taurine and glycine conjugates of these bile acids.
Bile acids are cholesterol metabolites that are formed in the liver and secreted into the duodenum of the intestine, where they have important roles in the solubilization and absorption of dietary lipids and vitamins. About 95% of bile acids are subsequently reabsorbed in the ileum and returned to the liver via the enterohepatic circulatory system. The conversion of cholesterol to bile acids in the liver is under feedback regulation, and bile acids down-regulate transcription of cytochrome P450 7A1 (CYP7A1), which encodes the enzyme that catalyzes the rate-limiting step in bile acid biosynthesis. FXR is involved in the repression of CYP7A1 expression by bile acids through an indirect mechanism involving the FXR target gene small heterodimer partner (SHP) and liver receptor homolog 1 (Goodwin et al., Mol. Cell 6:517-528, 2000; reviewed in Matsubara et al., Mol. Cell. Endocrinol. 368:17-29, 2013). In the ileum, in an FXR dependent manner, bile acids induce the expression of the intestinal bile acid binding protein (IBABP), a cytoplasmic protein which binds bile acids with high affinity and may be involved in their cellular uptake and trafficking. Two groups have now demonstrated that bile acids mediate their effects on IBABP expression through activation of FXR, which binds to an IR-1 type response element that is conserved in the human, rat, and mouse IBABP gene promoters. Thus, FXR is involved in both the stimulation (IBABP) and the repression (CYP7A1) of target genes involved in bile acid and cholesterol homeostasis. FXR also induces expression of the bile salt export pump (BSEP, ABC11) that transports unconjugated and conjugated bile acids/salts from hepatocyte into the bile (reviewed in Matsubara et al., Mol. Cell. Endocrinol. 368:17-29, 2013).
Tempol (4-hydroxy-2,2,6,6,-tetramethylpiperidine-1-oxyl), an antioxidant and a radiation protector, was reported to prevent obesity in mice (Mitchell et al., Free Radic. Biol Med. 34: 93-102, 2003). A recent mass spectrometry-based investigation revealed that tempol can affect fatty acid metabolism and the altered levels of suspected gut microbe-generated metabolites provided initial clues that tempol may alter the microbiome (Li et al., J. Proteome Res., 12:1369-1376, 2013). Previous studies demonstrated that the alteration of the gut microbiome can affect the level of bile acids in liver, heart, and kidney (Swann et al., Proc. Natl. Acad. Sci. USA 108:4523-4530, 2011). High fat diets can induce changes in the expression of genes in the small intestine that are controlled by nuclear receptors including FXR (de Wit et al., BMC Med. Genomics 1:14, 2008). Thus, there may be relationship between altered bile acids in the intestine and FXR signaling that can alter high fat diet-induced obesity. While there are known natural and synthetic FXR agonist, no therapeutic agents have been disclosed which antagonize FXR. Recent studies revealed that the secondary bile acid tauro-β-muricholic acid (TβMCA) can antagonize bile acid signaling in the intestine (Sayin et al., Cell Metab. 225-235, 2013; Li et al., Nat. Commun. 4:2384, 2013). Trisubstituted-pyrazol carboxamide analogs have been synthesized that are FXR antagonist, but their effect on metabolism and physiology were not investigated (Yu et al., Bioorg. Med. Chem. 2919-2938, 2014).
Non-alcoholic fatty liver disease (NAFLD) is characterized by massive ectopic triglyceride (TG) accumulation in the liver in the absence of other liver disease or significant alcohol consumption (WeiB et al., Ditsch. Arzteb.l Int. 2014; 0.447-452, 2014). NAFLD is the most common liver disorder affecting 20-30% of the adult population and more than 80% of obese people in the world. NAFLD can develop into nonalcoholic steatohepatitis (NASH), fibrosis, cirrhosis and even hepatocellular carcinoma (Browning et al., J. Clin Invest. 114:147-152, 2004). As a component of metabolic syndrome, NAFLD is tightly associated with obesity, insulin resistance/type 2 diabetes, and coronary heart disease and atherosclerosis (Bhatia et al., Eur. Heart 33:1190-1200, 2012). To date, the underlying molecular mechanism of NAFLD development remains largely unknown and the identification of novel targets for NAFLD therapy is of high priority.
The foregoing shows that there is an unmet need for antagonists of the FXR receptor and a method of treating obese patients to induce weight loss, insulin resistance, and NAFLD.